1. Field of the Invention
The present invention relates to an adjustable damping force shock absorber for a vehicle or the like, capable of varying the damping force in accordance with road conditions while the vehicle is running.
2. Description of the Prior Art
Hydraulic shock absorbers for vehicles or the like, capable of properly varying the damping force according to the road conditions to ensure a satisfactory ride and stable control are disclosed in, for example, Japanese Patent Publication Nos. 59-22359 and 60-37477. Such an adjustable damping force shock absorber has a cylinder having two chambers partitioned by a piston and communicating with each other by means of a plurality of passages. The shock absorber is provided with damping force generating means such as orifices which control the flow of the working fluid through the passages caused by the axial movement of a piston in a cylinder to produce a damping force, and varies its damping force by selectively opening or closing the passages by means of solenoid valves. Each of the solenoid valves is a normally closed valve having a valve element normally biased to a closed position by a coil spring or the like to keep the passage closed. The solenoid is energized to move the valve element away from the closed position against the resilience of the coil spring by magnetic attraction to open the passage.
This known adjustable damping force hydraulic shock absorber, however, has the following problems.
As for the foregoing known adjustable damping force hydraulic shock absorber, the pressure of the working fluid acts directly on the valve element. Therefore, the valve element must be biased to the closed position by a comparatively strong coil spring or the like, and hence the solenoid must be large and must be capable of exerting a high degree of attraction on the valve element, which increases the overall size of the hydraulic shock absorber. Furthermore, since the solenoid must be continually energized when keeping the solenoid valve open, power consumption is increased, the load on the battery increases, and heat generated by the solenoids of the solenoid valves accelerates the deterioration of the working fluid sealed in the hydraulic shock absorber.